Virus inoculum preparation.

The “atypical PRRS abortion storm” virus isolate NADC-20 (82) was passaged once in the porcine alveolar macrophage cell line ZMAC to create the challenge virus stock as previously described (83). Briefly, a cell-free preparation of the virus was prepared by centrifuging the medium overlaying the infected cell monolayers at 4°C and 350 × g for 10 min. Aliquots of the virus were stored at −80°C, and a sample of the virus stock was titrated for the presence of infectious virus. The virus titer was calculated using the Reed and Muench method (84) and expressed as TCID₅₀ per milliliter.